The present invention generally relates to soft contact lenses and a method of designing such lenses. More specifically, the present invention relates to a soft contact lens and the design of such a lens using novel methods of corneal topographic analysis.
The curvature of an unflexed soft contact lens, such as a lens placed in physiological saline solution. is different than the curvature of the same lens placed on the eye. This change in curvature is often referred to as flexure. (See. e.g., A. G. Bennet. xe2x80x9cPower Changes In Soft Contact Lenses Due To Bendingxe2x80x9d, The Ophthalmic Optician, 16:939-945. 1976, the contents of which are incorporated herein by reference). In the case of thin soft lenses placed on a typical eye, this change in curvature does not substantially affect the lens power. However, in the case of thick lenses of high positive power, bifocal soft lenses, or for subjects with corneal abnormalities (e.g., due to keratoconus), the change in power due to flexure may be significant.
The present invention matches the topography of the cornea to a corresponding topography of a soft contact lens. A geometrical transformation is used which maps the corneal elevation onto the back surface of an unflexed soft contact lens in such a way that the error induced by flexure of the lens is minimized. The mapping transformation takes into account the effects of flexure. The resulting contact lens has a back surface having a curvature which matches the specific elevations of the cornea, while the front surface can be spherical or any desired symmetrical or asymmetrical shape.
According to the present invention, a geometrical transformation is used which maps the corneal elevations, measured by a videokeratoscope, for example, into the back surface of an unflexed lens. The mapping is performed in such a way that the error introduced by flexure is minimized. The approach of the present invention utilizes a number of simplifications in achieving the desired lens design. The first simplification is that corneal elevations (i.e., difference away from an underlying best fit sphere) are taken as being much smaller than the apical radius of curvature of the cornea. The second simplification is that the lens material, when flexured, is uniformly deformed and all points on the lens stay in the same azimuthal angle. These simplifications help in achieving a practical engineering solution to the design of such lenses.
The mapping procedure is performed in two steps. First, the elevations of the cornea are mapped to a larger scale surface having a radius of curvature corresponding to that of an unflexed soft contact lens. Second, the scaled up elevation information is scaled down using an area preserving transformation.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the features of the present invention.